1. Field of the Invention
This invention relates generally to sound image processing for reproducing audio signals over headphones and, more particularly, to apparatus for causing the sounds reproduced over the headphones to appear to the listener to be emanating from a source outside of the listener's head and also to permit such apparent sound location to be changed in position.
2. Description of the Background
In view of the generally crowded nature of modern society, headphones and small earphones have been becoming more and more popular in providing personal musical entertainment. In addition, headphones are frequently used when playing video games when other are in the room. Although many headphones provide very good fidelity in reproducing the original sounds and also provide generally good stereo effects, such stereo effects really are based on sounds being either directly at the left ear or the right ear. In balanced signals, such as a monaural signal, where the signal at each ear is approximately the same, the sound will appear to the listener to be originating from a source at the center of his head. This is not considered a generally pleasant experience and is fatiguing to the listener after a short period of time.
This in-the-head sound placement is not present when reproducing sounds using loudspeakers placed in front of the listener such as found in a conventional stereo system. Moreover, the sound locations are presently being spread around the entire room in the so-called surround-sound systems. In these kinds of loudspeaker installations, good stereo imaging can be readily accomplished. Not only is good stereo imaging generally available with a pair of loudspeakers, but recent advances in digital signal processors have permitted digital filtering to be applied to audio signals to selectively position the apparent sound origins even outside of the fixed locations of the two stereo speakers. In other words, transfer functions are available to selectively locate a sound origin and by sequentially selecting such transfer functions it is possible to create virtual sound image locations that appear to move relative to the stationary listener.
Even though such systems are apparently made possible due to the human physiology, applying the same transfer functions used in the loudspeaker application to headphones has not resulted in acceptable results. Moving locations are not possible except the extremes from the left ear to the right ear, or vice versa, and more times than not the sound image still remains inside the listener's head. Quite probably this non-correlation between headphones and loudspeakers is due to the manner in which the human brain interprets the different times of arrival and different amplitudes of audio signals at the respective ears of the listener.
Therefore, a system that can provide an apparent or virtual sound location out of the headphone user's head is highly desirable and, moreover, a system in which the apparent sound source could be made to move, preferably at the instigation of the user, would also be highly desirable.